The providing of writing and document paper with so-called watermarks has been known for a long time. Such watermarks in principle are invisible but appear clearly when the sheet of paper provided with watermark is held up against the light or light is transmitted through it in some other manner. Traditionally watermarks are produced by impressions in, or contact with, strongly hydrated paper pulp distributed on the screen of a paper machine. The operation is carried out with the help of a so-called dandy roll.
Such watermarks are expensive to produce if the watermarked material is not to be manufactured in large quantities. As a rule, it is not economically justifiable to provide private note-paper with special watermarks using this prior practice.
However, there is a possiblity of producing so-called false watermarks (marks of watermark character detectable or made visible in transmitted light or radiation) in a chemical manner. These "watermarks" are produced in that the optical refractive index of the paper is altered locally by applying a chemical substance, for example a polymerizable substance, to the paper in the desired pattern. However, it has been found that these false watermarks are often quite visible without any transmittance of light and that they give the impression of a "grease mark" having been made on the paper.